The electrical activity of the heart is typically measured at the body surface by a set of electrodes. Two or more electrodes are combined to form a lead (e.g. limb leads are measuring the potential difference at two electrodes, precordial leads are measuring the potential difference of one precordial electrode to the central terminal, which is calculated as mean value of the potential measured at the limb electrodes). A lead measurement may be interpreted as a projection of the hearts equivalent dipole onto the lead vector where the origin of that dipole is located somewhere in the center of the heart.
The widely used standard 12-lead ECG thus provides 12 different views onto the hearts electrical activity, where the view directions (i.e. the lead vectors) are considered as known. The precordial leads e.g. correspond to vectors pointing from the center of the heart to the electrode position and the limb leads correspond to the edges of the well-known Einthoven's triangle ([5], section 15). These lead vectors however are only rough estimates and they do not account for subject specific conditions like body geometry or tissue conductivity. Furthermore the standard 12-lead ECG does not reveal direct information about the origin of the electrical activity within the heart. Typically, localization information is derived indirectly by interpretation of ECG forces, i.e. dipole momentum characteristics.
Many different lead sets have been proposed. Vectorcardiography (VCG) e.g. intends to arrange electrodes in a way, that the resulting lead vectors are approximately orthonormal ([3], [6]). However these lead configurations have been determined with help of a torso model, which also does not care for subject specific body conditions.
Attempts have been made to map standard 12-lead ECG measurements onto the VCG lead set, using a fixed linear transformation matrix [1], [2]. This approach is equivalent to modeling a non-moving dipole using standard 12-lead measurements where all body and measurement related conditions are considered fixed and given.
Attempts have been made to apply a single moving dipole model onto the electrical activity of the heart [9]. However those methods rely on given data or assumptions about the true heart position, body shape, tissue inhomogeneity and the real electrode positions.
Electrocardiographic imaging methods try to determine the potential distribution at the heart by solving the so-called inverse problem of electrocardiography ([4], [8]). They may use larger sets of dipoles to describe the electrical activity of the heart. However those individual dipole locations are fixed within the heart and the whole body geometry has to be obtained by other (expensive) measuring devices (e.g. CT scan, MRI or X-ray). Furthermore those approaches typically require a larger number of electrodes compared to the standard 12-lead ECG.
ECG measurements taken at the body surface are influenced by the location of the source of the electrical activity within the heart, by body geometry, tissue conductivity, real electrode positions and other non-cardiac electrical sources (e.g. muscle activity, electrical fields created by electrical devices). This hinders the comparison of ECG measurements and makes it difficult to determine global parameters for the identification of heart diseases (e.g. ST elevation or depression, assessment of T-wave properties).
It follows that there is a need for a method which allows to determine the electrical activity of a heart of a subject with improved precision, using standard ECG measurement devices, taking into account subject and measurement related conditions without requiring extensive measurements of the body characteristics of the subject in question.
To solve this problem a method is herein disclosed that describes the electrical activity of the heart in terms of a 3-dimensional process using measurements from body surface potentials and some weak prior knowledge about subject and measurement related conditions, where those subject and measurement related conditions are adjusted on basis of the data acquired during the body surface potential measurements.